<h2>DESCRIPTION</h2>

<em>v.lidar.correction</em> is the last of three steps to filter LiDAR
data. The filter aims to recognize and extract attached and 
detached object (such as buildings, bridges, power lines,  trees, etc.) 
in order to create a Digital Terrain Model. 
<br>
<br>
The module, which could be iterated several times, makes a comparison 
between the LiDAR observations and a bilinear spline interpolation with 
a Tychonov regularization parameter performed on the TERRAIN SINGLE PULSE 
points only. The gradient is minimized by the regularization parameter. 
Analysis of the residuals between the observations and the interpolated 
values results in four cases (the next classification is referred to that 
of the v.lidar.growing output vector):
<br>
<br>
<b>a)</b> Points classified as TERRAIN differing more than a threshold
    value are interpreted and reclassified as OBJECT, for both single and 
    double pulse points.
<br>
<br>
<b>b)</b> Points classified as OBJECT and closed enough to the
    interpolated surface are interpreted and reclassified as TERRAIN, for
    both single and double pulse points.

<p>
The length (in mapping units) of each spline step is defined by 
<b>ew_step</b> for the east-west direction and <b>ns_step</b> for the 
north-south direction.

<h2>NOTES</h2>

The input should be the output of <em>v.lidar.growing</em> module or the 
output of this <em>v.lidar.correction</em> itself. That means, this module 
could be applied more times (although, two are usually enough) for a better 
filter solution. The outputs are a vector map with a final point classification 
as as TERRAIN SINGLE PULSE, TERRAIN DOUBLE PULSE, OBJECT SINGLE PULSE or 
OBJECT DOUBLE PULSE; and an vector map with only the points classified as 
TERRAIN SINGLE PULSE or TERRAIN DOUBLE PULSE.

The final result of the whole procedure (<em>v.lidar.edgedetection</em>,
<em>v.lidar.growing</em>, <em>v.lidar.correction</em>) will be a point
classification in four categories:
<br>
<br>
TERRAIN SINGLE PULSE (cat = 1, layer = 2)
<br>
TERRAIN DOUBLE PULSE (cat = 2, layer = 2)
<br>
OBJECT SINGLE PULSE (cat = 3, layer = 2)
<br>
OBJECT DOUBLE PULSE (cat = 4, layer = 2)


<h2>EXAMPLES</h2>

<h3>Basic correction procedure</h3>
<div class="code"><pre>
v.lidar.correction input=growing output=correction out_terrain=only_terrain
</pre></div>

<h3>Second correction procedure</h3>
<div class="code"><pre>
v.lidar.correction input=correction output=correction_bis terrain=only_terrain_bis
</pre></div>


<h2>SEE ALSO</h2>

<em>
<a href="v.lidar.edgedetection.html">v.lidar.edgedetection</a>,
<a href="v.lidar.growing.html">v.lidar.growing</a>,
<a href="v.surf.bspline.html">v.surf.bspline</a>,
<a href="v.surf.rst.html">v.surf.rst</a>,
<a href="v.in.lidar.html">v.in.lidar</a>,
<a href="v.in.ascii.html">v.in.ascii</a>
</em>


<h2>AUTHORS</h2>

Original version of program in GRASS 5.4:
<br>
Maria Antonia Brovelli, Massimiliano Cannata, Ulisse Longoni and Mirko Reguzzoni
<br><br>
Update for GRASS 6.X:
<br>
Roberto Antolin and Gonzalo Moreno

<h2>REFERENCES</h2>

Antolin, R. et al., 2006. Digital terrain models determination by LiDAR 
technology: Po basin experimentation. Bolletino di Geodesia e Scienze 
Affini, anno LXV, n. 2, pp. 69-89.
<br>
<br>
Brovelli M. A., Cannata M., Longoni U.M., 2004. LIDAR Data Filtering and 
DTM Interpolation Within GRASS, Transactions in GIS, April 2004,  vol. 8, 
iss. 2, pp. 155-174(20), Blackwell Publishing Ltd.
<br>
<br>
Brovelli M. A., Cannata M., 2004. Digital Terrain model reconstruction in 
urban areas from airborne laser scanning data: the method and an  example 
for Pavia (Northern Italy). Computers and Geosciences 30 (2004) pp.325-331
<br>
<br>
Brovelli M. A. and Longoni U.M., 2003. Software per il filtraggio di dati 
LIDAR, Rivista dell'Agenzia del Territorio, n. 3-2003, pp. 11-22 (ISSN 1593-2192).
<br>
<br>
Brovelli M. A., Cannata M. and Longoni U.M., 2002. DTM LIDAR in area urbana, 
Bollettino SIFET N.2, pp. 7-26.
<br>
<br>
Performances of the filter can be seen in the
<a href="http://www.itc.nl/isprswgIII-3/filtertest/MainDoc.htm">ISPRS WG III/3 Comparison of Filters</a> 
report by Sithole, G. and Vosselman, G., 2003. 

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